1. Field of Invention
This invention is directed to semiconductor laser diodes emitting in the ultraviolet region of the spectrum.
2. Description of Related Art
In recent years, gallium nitride-based compound semiconductors, such as GaN, InGaN, GaAlN, and InGaAlN, have received a great deal of attention as materials for violet- and blue-emitting semiconductor lasers. Semiconductor lasers using these materials in the multi-quantum well (MQW) active region are designed as short-wavelength laser sources. Hence, their output can be focused to small diameters. Owing to this advantage, these lasers are expected to be usable as light sources for high density information storage, such as in an optical disk.
Because of their compactness, low power consumption and longevity, UV semiconductor lasers can also enable other applications that are unrealizable by solid state or gas lasers. Such applications can include sources for compact bioagent detection systems, for optical biopsy by exciting fluorescence in human tissue, in dentistry (UV curing), and in ophthalmology (to detect some kinds of eye damage). UV lasers might also be of interest in biotechnology applications such as cross linking nucleic acids with polymers, exciting DNA or proteins, and more.
However, GaN materials systems present unique problems in terms of realizing a UV laser. In particular, large polarization fields exist for example, within the In(Al)GaN and (In)AlGaN layers of a multi-quantum well device. The polarization fields arise from the large pyroelectric and piezoelectric constants in these materials. The polarization charges at the heterostructure interfaces create polarization fields that spacially separate the negatively charged electrons from the positively charged holes. For example, for an InGaN film with 10% In, on a GaN template, the polarization field in the quantum well can be in the range 1.5 MV/cm. These polarization fields tend to separate the wavefunctions of the electrons and holes within a quantum well, which in turn tends to reduce the wavefunction overlap and thus the optical gain of the laser device.
Polarization effects are particularly detrimental for In(Al)GaN/(In)AlGaN multi-quantum wells, where the fields cause not only uneven carrier distribution within but also uneven carrier distribution between the quantum wells.
The high-band gap (In)AlGaN barriers between quantum wells (QW) add further to the uneven carrier distribution in the multi-quantum well (MQW) active region. High-band gap (In)AlGaN barriers are normally required to prevent carriers from leaking out of the quantum wells. However the high barriers will also prevent carriers from moving easily between quantum wells and therefore make it difficult to achieve a even distribution of carriers between quantum wells.
Unevenly distributed carriers lead to uneven gain, which results in higher threshold current densities and lower quantum efficiencies. In this case, the carrier distributions can be so non-uniform that one or more of the quantum wells in the active region is below the transparency threshold, which can even quench lasing.
The problem of uneven carrier distribution in the active region can be reduced by constructing single quantum well (SQW) devices, because the PN junction is directly placed at the quantum well. Since there is only one quantum well, uniform carrier injection is not a problem. In addition, the single quantum well (SQW) active region has a smaller volume compared to the multi-quantum well active region, so that fewer carriers are needed to produce gain. Therefore, the threshold current for lasing with a single quantum well active region is normally lower than that for an multi-quantum well active region, provided that the loss in the laser is low enough.
Alternatively one could also use a closely-spaced coupled quantum well (CQW) approach. In this case the barriers between the quantum wells are fairly thin and/or have a relatively low band gap-energy to provide sufficient coupling between the quantum wells. The carriers in the quantum wells can then easily redistribute either by tunneling between quantum wells or by a thermally activated process that allows them to cross the barriers. The PN junction including larger bandgap confinement layers is placed adjacent to the coupled quantum well region to allow efficient carrier injection and to prevent carrier leakage. This approach allows a higher gain, which might be important for high power operation or necessary if the loss in the laser is elevated.